The present invention relates generally to a device and method for suppression of acoustical feedback. More particularly, the present invention relates to an automatically tunable notch filter and method for suppression of acoustical feedback.
Since the inception of electrical audio amplification systems, system instabilities have been known to result from the many varied acoustical paths between microphone and speaker. These system instabilities, otherwise known as acoustical feedback, result in electrical signals which, if permitted to go unattenuated, obliterate the desired audio signal.
A notch filter (also known as a band reject filter) is a well known device for substantially attenuating electrical signals between any two specified frequencies while not appreciably affecting signals at other frequencies outside this band. It is known that a notch filter tuned to a center frequency equal to a feedback frequency may be utilized for suppression of the feedback occurring at that frequency by holding the amplitude of the feedback signal to an acceptable level not conductive to feedback.
However, as a result of the nature of direct and multiple reflected acoustical paths between microphone and speaker, the frequency of acoustical feedback is unpredictable and may extend over a substantial portion of the audio frequency spectrum, which spectrum extends from frequencies of approximately 20 to 20,000 Hz. Therefore, in the past devices for suppression of acoustical feedback employed a plurality of discrete analog notch filters each selectively tuned to a portion of the audio frequency spectrum. Fortunately, the nature of acoustical feedback in general is such that at any given instant in time the acoustical feedback occurs at substantially a single frequency -- the instantaneous dominant feedback frequency.
In one device the notch filters are individually sequencially interposed between an audio amplifier and a loudspeaker by manual switching until the acoustical feedback is eliminated by the fortuitous selection of the filter which suppresses that frequency at which the acoustical feedback is then occurring. As a result of the necessity for manual switching in a plurality of filters, these devices have a very slow response time, are expensive and occupy large spaces due to the large number of discrete filters required. Furthermore, because of the fact discrete filters are required, only a limited number of filter frequencies are obtainable, necessitating that each filter have a bandwidth substantially broader than desirable, thereby rendering the filtered audio signal of poor quality. Finally, because the feedback frequency may only be determined by experimentally testing one filter at a time, only one portion of the audio frequency spectrum may be manually surveyed at any given time for the feedback frequency. Because the acoustical feedback paths are likely to change very rapidly, it is entirely possible for the feedback frequency to change several times in the course of just one complete "survey" of all the filters and ultimately result in entirely missing the feedback frequency at any given instant in time.
More recently, spectrum analyzers have replaced the sequential searching aspect of the manual switching technique just described. Here, the spectrum analyzer can continually survey the audio frequency spectrum, still one frequency at a time, and provide an indication of the amplitude of the signal at each frequency. Upon detecting a signal of an amplitude greater than a predetermined level, the operator may manually switch in the appropriate filter which suppresses that frequency. Alternately, conventional control circuitry may be provided to automatically switch in the appropriate filter.
Because the spectrum analyzer still must survey one frequency at a time, this device is also subject to an undesirable slow response time. Moreover, the bulky nature and high cost of spectrum analyzers, and remaining necessity for using a plurality of discrete notch filters, further contribute to the existing deficiencies of the manual searching device.
Generally, most audio signals one may be desirous of amplifying are non-periodic in nature. This is especially true in the case of human speech: rarely, if at all, does a person repeat exactly the same sounds one immediately after the other. In the presence of acoustical feedback the audio signal is overridden by a periodic signal having a frequency substantially equal to the dominant feedback frequency and a substantially sinusoidal shape with the audio signal superimposed thereupon. Heretofore, there has been no recognition or application of this principle in the art of acoustical feedback suppression.
By definition, a periodic signal repeats itself in constant time intervals known as periods. Although a period may be taken to begin at any point in time, for convenience we may begin and end a period each alternate time the amplitude of the input signal is substantially zero (after allowing for everpresent electrical noise), which amplitude is referred to in the art as the "zero-crossing level."
Since generally audio signals are non-periodic, in the absence of acoustical feedback successive periods are generally unequal. On the other hand, in the presence of acoustical feedback a periodic signal occurs resulting in successive periods being substantially equal. I have found that by monitoring and comparing successive periods in an audio signal the presence of acoustical feedback and the instantaneous dominant frequency thereof may be quickly, accurately and precisely determined for use in an acoustical feedback suppression apparatus.